Bioinspired Synthesis of Photocatalytic Nanocomposite Membranes

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Bio-inspired Synthesis of Photocatalytic Nanocomposite Membranes based on Synergy of Au-TiO2 and Polydopamine for the Degradation of Tetracycline under Visible Light Chen Wang, Yilin Wu, Jian Lu, Juan Zhao, Jiuyun Cui, Xiuling Wu, Yongsheng Yan, and Pengwei Huo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04902 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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ACS Applied Materials & Interfaces

Bio-inspired

Synthesis

of

Photocatalytic

Nanocomposite

Membranes based on Synergy of Au-TiO2 and Polydopamine for the Degradation of Tetracycline under Visible Light Chen Wang,a Yilin Wu,a Jian Lu,b Juan Zhao,a Jiuyun Cui,a Xiuling Wu,c Yongsheng Yan,a,* Pengwei Huoa,*

a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P. R.

China b

School of Chemistry, Jilin Normal University, Siping, 136000, P. R. China

c

School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou, 510006, P. R. China

Corresponding Author Email: [email protected]; [email protected]

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ABSTRACT A bio-inspired photocatalytic nanocomposite membrane was successfully prepared via polydopamine (pDA)-coated poly (vinylidene fluoride) membrane (PVDF) as a secondary platform for vacuum filtration Au-TiO2 nanocomposites with enhanced photocatalytic activity. The degradation efficiency of the Au-TiO2/pDA/PVDF membranes reached to 92% when exposed to visible light for 120 minutes. And the degradation efficiency of the Au-TiO2/pDA/PVDF membranes increased by 26% compared with that of Au-TiO2 powder, and increased by 51% compared with that of the TiO2/pDA/PVDF nanocomposite membranes. The degradation efficiency remained about 90% after five cycle experiments, and the Au-TiO2/pDA/PVDF nanocomposite membranes showed good stability, regeneration performance and easy recycling. The pDA coating not only was served as a bioadhesion interface to improve the bonding force between the catalyst and membrane substrate, but also acted as a photosensitizer to broaden the wavelength response range of TiO2. And the structure of Au-TiO2/pDA/PVDF also improves the transfer rate of photogenerated electrons. And the surface plasmon resonance effect of Au also played a positive role in improving the activity of the catalyst. Therefore, we believe that this study opens up a new strategy in preparing the bio-inspired photocatalytic nanocomposite membrane for potential waste water purification, catalysis and the membrane separation field. Keywords: photocatalytic membranes; polydopamine; Au-TiO2 nanocomposite; tetracycline; visible light degradation

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1. INTRODUCTION Tetracycline, a highly effective broad-spectrum antibiotic,1 is the indispensable part of maintaining the health of humans and animals,2 which can treat a variety of diseases including mycoplasmal pneumonia and epidemic typhus.3 Recently, the sustained emissions and environmental accumulation of tetracycline has aroused widespread concern in society.4,5 The residual of tetracycline in the environmental can selectively kill some microbes, while can induce the production of some drug-resistant bacteria thus endangering human health and aquatic ecosystem balance.6-8 At present, researchers have studied the removal technology of tetracycline, such as adsorption, microbial degradation, electrolysis, photocatalytic oxidation and membrane separation.9-11 Among them, photocatalytic oxidation technology12-14 can efficiently degrade organic matter because of its strong oxidizing ability and utilize solar energy,15 which has become a research hotspot in recent years. Developing stable, easily recyclable and high active photocatalyst materials that can sufficiently absorb visible light is of great significance for photocatalytic reaction process. Some scholars have been focused on the research of semiconductor photocatalysis and photoelectron-catalysis since the Honda-Fujishima effect emerged in 1972, which has led to the rapid development of semiconductor photocatalysis.16-18 At present, numerous semiconductor materials were reported included ZnO19, TiO220, Bi2O321, V2O522 and CeO223. Between them, TiO2 was considered as a promising photocatalyst for industry and environmental protection applications24 because of its chemical stability, excellent photocatalytic activity, low-cost, easy access and environmentally friendly nature.25 However, the wide band gap of TiO2 (3.0-3.2 eV) restricted the spectral response range (pristine TiO2 only absorbed UV light), and the photogenerated electron-hole recombination

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rate of pristine TiO2 was fast, which led to the decrease of its catalytic activity.26 It was pointed out that doping noble metal nanoparticles (Ag, Au, Pt and Pd) was an effective means to enhance the optical absorption due to surface plasmon resonance effect and accelerate the interfacial charge transfer of the catalyst.27,28 For instance, the Au-TiO2 nanocomposites exhibited higher photocatalytic activity than Au nanoparticles and pure TiO2 nanoparticles.29-32 However, the nanocatalysts without a stable support could easily aggregate in solution, which always resulted in reduction of their catalytic activity. It is difficult to recycle these nanocatalysts from the reaction system because of their small sizes. So with that being said the recovery and reuse of the powder catalyst is still a technical difficulty. To address the above problem, some scientists proposed that porous membranes (e.g., renewable cellulose membrane, polycarbonate membrane, porous ceramic membrane) could be served as an ideal support to immobilize various nanoparticles.33,34 Immobilizing the nanocatalyst to poly (vinylidene fluoride) membrane (PVDF) can effectively improve the stability and recovery of the catalyst. In particular, the PVDF membrane has excellent comprehensive properties including high mechanical strength, good chemical stability and UV radiation resistant.35 However, it is absolutely necessary to overcome the weak bonding between the catalyst and the membrane. Messersmith and co-workers found dopamine (DA) in the marine mussel protein, and DA as a functional biomaterial was often served for an excellent surface-adherent material.36-39 The self-polymerizing dopamine layer can not only strengthen bonding force between nanocatalyst and membrane to increase the stability of the photocatalytic membrane, but also can be viewed as a photosensitizer and an electron donor to increase the visible response range.40-44 Therefore, it is

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highly desirable to incorporate Au-TiO2 nanocomposite materials on pDA-coated membrane for the degradation of tetracycline. In this paper, we prepared a bio-inspired photocatalytic nanocomposite membrane structure for tetracycline degradation under visible light irradiation. A vacuum filtration procedure45 was carried out to filtration the Au-TiO2 nanocomposites prepared by sodium citrate reduction, ambient temperature reaction and hydrothermal methods onto the pDA-coated PVDF membrane (Au-TiO2/pDA/PVDF). The catalytic performance of the photocatalytic nanocomposite membrane was determined by UV-vis spectrophotometer, and the structure and properties of the prepared materials can be obtained by the relevant characterization means of field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), dynamic contact angle measuring device/tensiometer, X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR), UV-Vis absorption/diffuse spectra (DRS) of the solid and FTIR-Fourier Transform Infrared Spectrometry, etc. This work proposes an approach to prepare the photocatalytic membrane, and it is a significant step in the study of powder catalyst recovery.

2. MATERIALS AND EXPERIMENTAL METHODS 2.1. Materials. Poly (vinylidene fluoride) membrane (PVDF, 0.22 µm average pore size, diameter of 25 mm) were supplied by Sartorius and used as the catalyst support in all experiments. Tetracycline (98%, Aladdin), Tris (hydroxymethyl) aminomethane (Tris-HCl, 99%, Aladdin), dopamine (DA, 98%, Aladdin), Titanium isopropoxide (C12H28O4Ti, 95%, Aladdin), Gold chloride trihydrate (HAuCl4·3H2O, 99.9%, Aladdin), Sodium citrate dehydrate (C6H5Na3O7·2H2O, 99.0%, Aladdin), Isopropyl Alcohol (C3H8O, 99.9%, Aladdin), Ammonia solution (NH3·H2O, 25%, Aladdin), Ethanol (C2H5OH, 99.5%, Aladdin), Methyl alcohol (CH3OH, 99.5%, Aladdin),

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5,5-Dimethyl-1-pyrroline N-oxide (DMPO, C6H11NO, 97%, Aladdin), Deionized water was distilled by a Milli-Q system (Millipore, the resistivity: 18.2 MΩ·cm). All solvents and chemicals were used as received. 2.2. Experimental Methods. 2.2.1. Synthesis of Au nanoparticles colloidal solution. Au nanoparticles colloidal solution was synthesized by the sodium citrate dehydrate reduction method based on a previous report.30 Briefly, 0.0206±0.0003 g of HAuCl4·3H2O was dissolved in 50 mL deionized water and transferred to a 100 mL three-neck round-bottom flask. The mixture was reflux heated to 125 oC in an oil bath while stirred at a rate of 500 rpm. Then, 5 mL of sodium citrate dehydrate solution (38.8 mM) was injected in the reaction system as the reducing agent. The reduction process was inferred by a change of mixed solution color from pale yellow to purple red. After being continuously stirred for 10 min at 125 oC, turned off the temperature of the oil bath and naturally cooled down to 25 oC. Finally, the colloidal solution of Au nanoparticle was obtained. 2.2.2. Preparation of pure TiO2, Au-TiO2 nanocomposites. The Au nanoparticles colloidal (20 mL), isopropanol (40 mL) and ammonia (1.25 mL) were mixed to prepare for Au-TiO2 nanocomposites, where isopropanol was done as a dispersing agent for the disperse mixing of water and TiO2 precursor. Then, titanium isopropoxide (6 mL) was added into the above mixture and stirred constantly for 20 h at 25 oC. The obtained products were collected by centrifugation at a speed of 10000 rpm and washed three times with isopropanol and deionized water, respectively. Then the samples were dried under vacuum at 40 oC to obtain amorphous Au-TiO2 nanocomposites. The synthesized amorphous Au-TiO2 nanocomposites were used for a further hydrothermal treatment to obtain anatase Au-TiO2 nanocomposites. Typically, 0.2000±0.0005 g of the prepared Au-TiO2

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nanocomposites was dissolved in the mixing solution of ethanol (20 mL) and H2O (10 mL), and the mixture was ultrasonically dispersed for 30 min in an ultrasonic bath, and transferred into a Teflon-lined autoclave (50 mL) and heated at 200 oC for 5 h. The obtained products were collected by centrifugation at the speed of 10000 rpm and washed three times with ethanol and deionized water, respectively. As a comparison, anatase TiO2 nanoparticles were also prepared in the same manner as that of Au-TiO2 nanocomposites, except that the Au nanoparticles colloidal solution was substituted with deionized water. 2.2.3. Preparation of the Au-TiO2/pDA/PVDF and TiO2/pDA/PVDF nanocomposite membranes. A two-step prepared procedure of the Au-TiO2/pDA/PVDF membrane was described as follows: (1) the preparation of the pDA/PVDF membrane was based on a previous study.39,46 The original PVDF membranes were immerged 5 min at 100 mL Tris-HCl buffer solution (pH=8.5). In a parallel step, DA (2 mg / mL) was rapidly dissolved in 100 mL of Tris-HCl buffer solution and the pH of the solution was adjusted to 8.5 with dilute HCl or NaOH solution. Then, the preprocessed PVDF membranes were immersed in dopamine-containing buffer solution and persistently shaken in the air oscillator for 5 h at 25 oC to form pDA layer on the original PVDF membranes. In the end, the pDA/PVDF membranes were washed with deionized water three times and dried at 25 oC. (2) The Au-TiO2/pDA/PVDF nanocomposite membranes were prepared by vacuum filtrating method. 5 mL of Au-TiO2 aqueous solution was filtrated on the pDA/PVDF membrane with vacuum filtration installation (Figure S1 in the Supporting Information). Then, the prepared photocatalytic nanocomposite membranes were dried at ambient temperature. Likewise, the TiO2/pDA/PVDF nanocomposites membranes also can be prepared by the same method.

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2.2.4. Test of photocatalytic activity. For a typical photocatalytic experiment, the photocatalytic activity test of the Au-TiO2/pDA/PVDF nanocomposite membranes and the control experiments (the original PVDF membranes, pDA/PVDF membranes, Au-TiO2 powder and TiO2/pDA/PVDF nanocomposite membranes) were carried out with 20 mL of tetracycline solution (10 mg/L) at 30 o

C under visible light irradiation. Specifically, 5 pieces of Au-TiO2/pDA/PVDF nanocomposite

membrane or 15 mg Au-TiO2 powder were placed in degradation bottles with 20 mL tetracycline adsorption solution. And then the system was positioned in darkness condition for 1 h to achieve adsorption equilibrium in steady state. In the experiment, the xenon light source (distance: 10 cm, Yangzhou University City Technology Co., Ltd) with a power of 300 W was used to simulate the sunlight, and a filter was used to block the light in a wavelength of less than 420 nm. Then, the reaction system was directly exposed to the Xenon lamp source to degrade tetracycline in steady state. In the process of degradation, taking of samples was implemented at stated time intervals and the tetracycline concentrations were detected with a UV-vis spectrophotometer (spectral range: 190-1100 nm, the resolution: 0.1 nm). The tetracycline concentrations were calculated by determining

absorbance

of

tetracycline

at

the

maximum

absorption

wavelength.

Langmuir-Hinshelwood (L-H) model34 (the pseudo-first-order rate equation) was used to illustrate the photodegradation kinetic behavior shown as follows:

ln

C0 = kt C

(1)

Where C (mg/L) and C0 (mg/L) represent the concentration of tetracycline at different time during the process of photocatalytic degradation and the concentration of adsorption equilibrium, respectively; t (min) is the degradation time, and k (min−1) is the apparent rate constant.

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2.3. Characterization of Materials. The morphologies of the nanoparticles and membranes were observed by a Field Emission Scanning Electron Microscopy (SEM, Jeol, JSM-7001F) and Transmission Electron Microscopy (TEM, Jeol, JEM 2100 (HR)). X-ray diffraction (XRD, Rigaku, Smartlab, operated at 40 kV and 200 mA, Cu Kα source) was used to determine the crystal structure. Dynamic contact angle measuring device/tensiometer (Germany Dataphysics Company, DCAT11) was used to measure the hydrophilicity of the membranes. The X-ray photoelectron spectroscopy (XPS, ESCALAB 250X, America) was used to obtain the surface chemical composition of the samples. The UV-vis absorption/diffuse spectra (DRS) were performed using a Shimadzu UV-vis 2550 spectrophotometer. BaSO4 was used as a reflectance standard material. The Atomic Force Microscope (AFM, MFP-3D, America) was used to measure the roughness of the samples. The photoluminescence (PL) spectra was obtained on a F4500 (Hitachi, Japan) photoluminescence detector. The Fourier transform infrared spectra (FT-IR) of the materials were analyzed using Nicolet Nexus 470 spectrometer. The degradation efficiency of the target was detected by a UV-vis spectrophotometer (Agilent, USA, Cary 8454).

3. RESULTS AND DISCUSSION 3.1. Morphology and structure of TiO2, Au-TiO2 nanocomposites. The Au nanoparticle colloidal was prepared by sodium citrate reduction method. The amorphous Au-TiO2 nanocomposites were produced by room temperature reaction (stirring for 20 h at 25 oC), and the Au-TiO2 nanocomposites with anatase crystal were prepared by a mild method of hydrothermal treatment at 200 oC for 5 h. Compare with high temperature calcination method,31,47,48 the hydrothermal method did not require grinding and roasting, and was easier to control the morphology of the sample. Figure 1 showed TEM images of TiO2 nanoparticles, Au nanoparticles,

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and Au-TiO2 nanocomposites. As TEM images depicted in Figure 1a-b, the TiO2 nanoparticles were spherical-like structure with 20-25 nm in diameter, and Au nanospheres with a diameter of 15 nm. The results illustrated that these nanoparticles prepared by the above-mentioned methods could provide more uniform particle size distribution. As shown in Figure 1c, there were differences in TiO2 edges presented in the composite material, which suggested Au nanoparticles homogeneously recombined onto the surface of the TiO2 system (arrow referred to the material that was Au nanoparticles). As shown in Figure 1d, a selected area electron diffraction (SAED) pattern shown several strong Debye-Scherrer rings in the investigated sample corresponding to single-crystal anatase phase of TiO2, which was more favorable for catalytic reactions than rutile.49,50 The XRD patterns of pure TiO2 and Au-TiO2 powders were examined to study the crystal structure and composition, as shown in Figure 2. The TiO2 diffraction peaks of XRD patterns at 25.3o, 37.8o, 47.9o, 53.8o, 55.0o, 62.7o and 68.7o (2θ) can be indexed to the anatase TiO2, which can be respectively assigned to (101), (004), (200), (105), (211), (204), and (116) diffraction planes of the anatase phase of TiO2 (JCPDS No. 21-1272),51,52 which was consistent with the result of Figure 1d. The corresponding values between TiO2 diffraction peaks and diffraction planes were also listed in Table S1 (Supporting Information for details), and the result showed that the prepared pure TiO2 powder had a high crystallinity and a good match with the anatase TiO2. Moreover, the XRD patterns of Au-TiO2 powder showed the characteristic peaks of the gold nanoparticles at 38.22o and 44.40o (2θ) corresponding the previous report.53 It proved that gold nanoparticles were existed in the complex. The XRD patterns of the complex also illustrated that the addition of gold nanoparticles did not affect the crystallinity and activity of titanium dioxide.

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3.2. Characterization of membranes. The SEM images of membranes were compared in Figure 3a-d. As we have observed, the original film surface was porous and smooth. With the modification of the pDA and the loading of the catalyst, the original PVDF membrane surface gradually became rougher and the specific surface area became larger, implying that the pDA coating and the catalytic layer were fixed on the original PVDF membrane. The corresponding colors of the original PVDF, pDA/PVDF, TiO2/pDA/PVDF and Au-TiO2/pDA/PVDF membrane were white, brown, white and purple (insets in Figure 3a-d), respectively, suggesting the presence of pDA, TiO2 and Au-TiO2 on the PVDF membranes. Moreover, it can be clearly observed that the catalyst nanoparticles were present in the membrane surface and the pores (Figure S2). The hydrophilicity of the membrane was determined by measuring the water contact angle, as seen in Figure 3e-h and Figure S3. Compared with the original PVDF membrane, it can be found that the water contact angle of pDA-coated membrane got smaller owing to the presence of hydroxyl and amino groups on the pDA surface (Figure 3e and 3f). And it also can be seen that the

TiO2/pDA/PVDF nanocomposite membranes had a small water contact angle because of the presence of hydroxyl groups on the TiO2 surface (Figure 3g). However, the water contact angle of the Au-TiO2/pDA/PVDF membranes was larger than that of the TiO2/pDA/PVDF membranes because some hydroxyl groups of the TiO2 surface were destroyed or covered by gold nanoparticles (Figure 3h). The error values of the water contact angles on both sides of the samples were also shown in Figure S3. Therefore, the special structure of photocatalytic nanocomposite membrane not only made the catalyst evenly disperse on the membrane, but also greatly increased the hydrophilicity of the membranes, which could be effective to mitigate membrane pollution.

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The AFM optical photographs of different samples were shown in Figure 4. Figure 4a revealed the original PVDF was a porous surface. The presence of the coated layer can be clearly observed in Figure 4b. The result suggested that pDA was successfully coated on the surface of the membranes. Figure 4c and d respectively showed the appearance of the TiO2 and Au-TiO2 powder on the membranes. Moreover, the AFM data also shown that the surface roughness of the different membranes was 189.190 nm (original PVDF), 345.791 nm (pDA/PVDF), 276.473 nm (TiO2/pDA/PVDF) and 457.111 nm (Au-TiO2/pDA/PVDF), respectively. The results indicated that the surface of the original PVDF is relatively smooth. Fourier transformed infrared (FT-IR) spectra was used to prove the change of functional groups on the membrane surface. As shown in Figure 5, the pristine PVDF membrane at 1404 cm-1 exhibited a strong and sharp absorption signal, which was attributed to the stretching vibrations of C-H. The peak located at 1181 cm-1 was belonged to the vibration of C-F. Moreover, there were different intensity absorption peaks at 486.22, 615.35, 764.13, 795.54, 878.23 and 975.54 cm-1, which was attributed to the characteristic absorption peaks of α-phase PVDF. After pDA coating modification, there was a new peak at about 3100-3400 cm-1, which was correspond to the stretching of hydroxyl groups. The result certified that a pDA layer successfully coated onto the PVDF membranes, and it was in accord with the observation from SEM and water contact angle. Compare with pDA/PVDF membranes, the peak intensity of hydroxyl groups located at 3000-3400 cm-1 became stronger after filtering TiO2 onto the pDA/PVDF membranes, which was attributed to the hydroxyl groups on the surface of TiO2. And the new peak appeared at 1650 cm-1, which was connected with H-O-H water and hydroxyl deformations. The signals of TiO2 peak located at 500-700 cm-1. On the contrary, we clearly observed that the peaks signal at 3100-3400

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and 1650 cm-1 became weaken in the FT-IR spectrum of the Au-TiO2/pDA/PVDF membranes. The result confirmed the presence of Au destroyed or covered some hydroxyl groups, which reduced the hydrophilicity of the membranes. It was consistent with the results of the water contact angle. The X-ray diffraction (XRD) patterns of the different membranes were shown in Figure 6, respectively. The XRD pattern of the original PVDF membranes exhibited strong characteristic peaks at 17.5o, 18.6o, 19.9o and 26.6o (2θ), corresponded to (100), (100), (110) and (021) crystal plane of α-phase PVDF, respectively. The XRD result of the original PVDF membranes also confirmed the FT-IR result, and it was in conformity with the characteristic peaks of the reported PVDF crystals. For the XRD patterns of both the TiO2/pDA/PVDF and Au-TiO2/pDA/PVDF membranes, except for the characteristic peaks of the original PVDF membranes, there were three new peaks located at 25.3o (101) and 47.9o (200), which means that the TiO2 or Au-TiO2 nanocomposites immobilized on the original PVDF membranes was mainly composed of anatase phase TiO2. The Au nanoparticles characteristic peaks did not appear in XRD spectrum, which was attributed to the stronger diffraction peaks of PVDF than that of Au. Optical properties of pure pDA powder, pure TiO2 powder, Au-TiO2 powder, original PVDF membranes, pDA/PVDF membranes, TiO2/pDA/PVDF membranes, Au-TiO2/pDA/PVDF membranes and Au-TiO2/PVDF membranes were determined by the UV-vis absorption/diffuse spectra (DRS), and the results were showed in Figure 7. The pure pDA powder was prepared by shaking in the air oscillator for 5 h at 25 oC (Supporting Information for details). As described in Figure 7, the pure pDA powder had a positive UV-vis light response at 200-700 nm. The pure TiO2 and Au-TiO2 powder had the same absorption edge around 410 nm, and a sharp absorption peak at

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about 550 nm for Au-TiO2 powder was assigned to the surface plasmon resonance effect of Au. The original PVDF membranes did not absorb light between 250-700 nm, while the pDA/PVDF membrane showed wide absorption at 200-700 nm range. The results indicated that pDA had a positive response to UV-vis absorption. It have been found that the TiO2/pDA/PVDF membranes had a strong UV absorption of TiO2 at 200-400 nm and wide visible light absorption at 400-700 nm of pDA. Meanwhile, the spectrum of Au-TiO2/pDA/PVDF membranes had the sharp absorption peak at about 550 nm, which was corresponded to the surface plasmon resonance effect of Au. Compare with the Au-TiO2/pDA/PVDF membranes, the Au-TiO2/PVDF membranes had the plasma resonance absorption of Au at about 550 nm without visible light absorption between 400 and 700 nm, which also demonstrated the photosensitizer role of pDA. The above results indicated both pDA and Au played beneficial roles in the visible light driven photocatalytic degradation process, and TiO2 and original PVDF membranes were acted as an electron transport media and a support substrate, respectively. In addition, X-ray photoelectron spectroscopy (XPS) analysis was used to further study the chemical state of surface constituent elements of different synthesized membranes (Figure 8), and XPS data fitting information was described in the Supporting Information. As shown in Figure 8a, a new N 1s peak emerged after pDA modification, and the N-C, C-N-C, C-H bond were observed from the narrow scan of N 1s peak (Figure 8b), illustrating the grown of pDA layers on the surface of the original PVDF membranes. After the vacuum filtrating TiO2 or Au-TiO2 onto the pDA/PVDF membranes, we found out that the intensity of F 1s, N 1s, and C 1s peaks in the membranes reduced a lot but the intensity of O 1s peaks enhanced in comparison with the pDA/PVDF membranes (Figure 8a), which was attributed to the formation of the catalyst layer.

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Figure 8c showed the narrow scan for Ti 2p3 peaks. The emergence of Ti 2p3 new peaks implied the TiO2 or Au-TiO2 catalytic layers successfully formed on the pDA/PVDF membranes surface. Meanwhile, the binding energy of Ti 2p3 peak located at 459 eV, indicating that the chemical state of Ti4+ 2p3/2 existed in the nanocomposite membranes (Figure 8c). Additionally, Au 4f peak of the Au-TiO2/pDA/PVDF membranes was also observed directly in Figure 5a, which suggested Au and TiO2 successfully integrated together. And the Au 4f peak binding energy at 83.7 eV and 88.1 eV were assigned to Au 4f7/2 and Au 4f5/2 of metallic Au (Figure 8d). And furthermore, the binding energy of Au 4f7/2 showed a deviation compared to the 84.0 eV of bulk Au, and the deviation was ascribed to the interfacial interaction between Au and TiO2, which could played a crucial role in catalytic process. 3.3. Photocatalytic performance of the membranes. The photocatalytic performance of the Au-TiO2/pDA/PVDF nanocomposite membranes has been evaluated preliminarily by degrading tetracycline solution under visible light irradiation.51,54 As a comparison, the experiments of the original PVDF membranes, pDA/PVDF membranes, Au-TiO2 powders and TiO2/pDA/PVDF membranes were implemented under the same conditions. And first the adsorption experiments of the different samples were performed for tetracycline solution in darkness, as shown in Figure S4. Compare with the original PVDF membranes, the pDA/PVDF membranes showed the adsorption performance of 0.5% for tetracycline, suggested that the original PVDF and pDA/PVDF membranes were hardly adsorbed to tetracycline. The adsorption equilibrium was achieved at 60 min for the samples (Figure S4). And the maximum equilibrium adsorption percentage reached to 30% within 80 min for the Au-TiO2/pDA/PVDF membranes, 28% for the TiO2/pDA/PVDF membranes, and 34% for the Au-TiO2 powder. The different adsorption results between the

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catalytic membrane and the Au-TiO2 powder were attributed to the larger specific surface area of the powder. Before commencing the Xenon lamp, the system was placed in darkness for 1 h to reach adsorption equilibrium to further study photocatalytic performance. As described in Figure 9a, the pristine PVDF membranes and the pDA/PVDF membranes had hardly catalytic activity for tetracycline under visible light irradiation, which implied that the original PVDF membranes had no any photocatalytic activity. In contrast, the photocatalytic activities of the Au-TiO2 nanocomposite powder, TiO2/pDA/PVDF membranes, and Au-TiO2/pDA/PVDF membranes were significantly improved under visible light irradiation. The Au-TiO2/pDA/PVDF membranes owned 92% of photodegradation efficiency toward tetracycline within 120 min, 73% for the Au-TiO2 powder, and 61% for the TiO2/pDA/PVDF membranes at the same time (Figure 9a). The results confirmed that the synergy of Au-TiO2 and pDA was conducive to improve the degradation efficiency of tetracycline. The TiO2/pDA/PVDF membranes also exhibited catalytic activity for tetracycline by combining with pDA. The reason is that the possible interaction between TiO2 and pDA gives the TiO2/pDA/PVDF membranes a certain catalytic activity.40-42 In order to further prove the role of the pDA coating, the photoluminescence (PL) spectra55,56 was used to analyze the photoelectron transfer as shown in Figure S5. The spectral intensity of Au-TiO2/pDA/PVDF membranes was lower than that of Au-TiO2/PVDF membranes, which indicated that the photoelectron recombination rate of the Au-TiO2/pDA/PVDF membranes was reduced. This result was attributed to the interfacial interaction of TiO2 and pDA to accelerate electron transfer. Depth study of membrane photocatalytic performance, the pseudo-first-order reaction kinetics was fitted according to the equation of ln(C0/C)=kt (Figure 9b). The high coefficient of measurement (R2>0.9) shown that the catalytic reaction was well fitted to the pseudo-first-order reaction kinetics. The

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apparent rate constant of membranes (k) were calculated from Figure 9a. The calculated values of the apparent rate constant k were 0.02185min-1, 0.01134min-1, 0.00827min-1 for the Au-TiO2/pDA/PVDF

membranes,

Au-TiO2

powder, and

TiO2/pDA/PVDF

membranes,

respectively. Moreover, it can be calculated that the k value for Au-TiO2/pDA/PVDF membranes increased 93% and 164% than those for the Au-TiO2 powder and the TiO2/pDA/PVDF membranes (Figure 9b). The species of active radicals were detected by DMPO with Electron Spin Resonance (ESR). It had observed from Figure 10a, DMPO-•O2- was successfully detected in methanol, and the signal strength was strong. It confirmed that the •O2- was main reactive species in the process of photocatalytic degradation of tetracycline. On the contrary, the weak signals of •OH were found in Figure 10b, which suggested that •OH was not main active species in the process of Au-TiO2/pDA/PVDF photocatalytic degradation of tetracycline. Based on the above results, the photocatalytic mechanism was described in detail, as shown in Figure 11. Because of the surface plasmon resonance effect of Au, the photoinduced electrons (e-) were quickly injected into the conduction band (CB) of TiO2 from Au. Meanwhile, pDA absorbed visible light to produce photogenerated electrons that were transfer quickly into the CB bottom of TiO2.40-42 Then, these electrons on the CB of TiO2 captured O2 on the surface of the catalyst to produce active species •O2-. And the active species (•O2-) and photoinduced holes (h+) reacted directly with tetracycline to yield degradation products. The original PVDF membranes only played a substrate support role in the catalyst recovery and reuse. Therefore, the catalytic activity of the Au-TiO2/pDA/PVDF nanocomposite membranes has been significantly improved.

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3.4. Catalytic stability of the Au-TiO2/pDA/PVDF nanocomposite membranes. Recycling is crucial for nanocatalysts in practice. Therefore membrane renewability experiment was carried out by repeating photocatalytic degradation of tetracycline. After each experiment, the membranes were washed 30 min in water for the next catalytic experiment. Figure S6 intuitively showed the regeneration capacity of the Au-TiO2/pDA/PVDF membranes, in which the degradation efficiency of the Au-TiO2/pDA/PVDF membranes still maintained 90% at 120 min after five cycle experiments. The Au-TiO2/pDA/PVDF nanocomposite membranes have exhibited good stability and regeneration capacity in the process of degradation of tetracycline. For the AFM measurements of the Au-TiO2/pDA/PVDF membranes thickness before and after the degradation process were used to prove the stability of the membrane, as shown in Figure S7. AFM images suggested the Au-TiO2/pDA/PVDF membranes of before degradation process had a rough surface and a height fluctuation of 200-400 nm. And a height fluctuation of the Au-TiO2/pDA/PVDF membranes after the degradation process was about 200-380 nm (Figure S7), and only a little bit down. This result revealed that the Au-TiO2/pDA/PVDF membranes had a good stability. Typically, the good regeneration of the Au-TiO2/pDA/PVDF nanocomposite membranes can be attributed to the bioadhesion of pDA and the support of the base film, which facilitated the rapid recovery and uniform dispersion of the catalyst.

4. CONCLUSIONS We have successfully synthesized of a novel bio-inspired photocatalytic nanocomposite membrane. A two-step modification methodology (dopamine coating method and vacuum filtration process) was introduced on the porous PVDF membranes to obtain the Au-TiO2-based dopamine modified nanocomposite membrane structure for degrading tetracycline. The Plasmon

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resonance effect of Au nanoparticles played a positive part in enhancing photocatalytic activity under visible light irradiation. The pDA coating, as the bioadhesion interface and photosensitizer, also played an important role in the catalytic process. The catalytic activity of the nanocomposite membranes has been investigated by degrading tetracycline under visible light irradiation. The degradation ratio of the Au-TiO2/pDA/PVDF nanocomposite membranes reached to 92% within 120 min, and the degradation efficiency of the nanocomposite membranes increased by 26% compared with that of Au-TiO2 powder, and increased by 51% compared with that of the TiO2/pDA/PVDF nanocomposite membranes. The maximum equilibrium adsorption percentage of the Au-TiO2/pDA/PVDF membranes reached to 30% within 80 min for tetracycline. The nanocomposite membranes with Au-TiO2 and pDA demonstrated high hydrophilicity, catalytic activity and stable properties. And the nanocatalysts with base-membrane support are easily recyclable. Therefore, the bio-inspired photocatalytic nanocomposite membranes are expected to be used in these fields of potential wastewater purification, photocatalytic oxidation and membrane separation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The vacuum filtering installation photograph; XRD table of anatase TiO2 diffraction peaks and diffraction planes; SEM images of the Au-TiO2/pDA/PVDF membranes; water contact angle; methods for the synthesis of the pure pDA powder; XPS Fitting Information; the adsorption rate curves of the different samples; PL spectra of the Au-TiO2/PVDF and Au-TiO2/pDA/PVDF

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membranes; the regeneration performance of the Au-TiO2/pDA/PVDF membranes; AFM images and height profiles along the line of the Au-TiO2/pDA/PVDF membranes before and after degradation process.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. ORCID Chen Wang: 0000-0001-7083-3292 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Graduate Practice Innovation Program of Jiangsu Province (No. SJLX160431), the National Natural Science Foundation of China (Nos. U1507118, U1407123, 21406085, and 21676127), and the Natural Science Foundation of Jiangsu Province (Nos. BK20151350, BK20140580, BK20161367).

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(52) Wang, K. X.; Wei, M. D.; Morris, M. A.; Zhou, H.; Holmes, J. D. Mesoporous Titania Nanotubes: Their Preparation and Application as Electrode Materials for Rechargeable Lithium Batteries. Adv. Mater. 2007, 19, 3016-3020. (53) Zhang, J. M.; Jin, X.; Morales-Guzman, P. I.; Yu, X.; Liu, H.; Zhang, H.; Razzari, L.; Claverie, J. P. Engineering the Absorption and Field Enhancement Properties of Au-TiO2 Nanohybrids via Whispering Gallery Mode Resonances for Photocatalytic Water Splitting. ACS Nano 2016, 10, 4496-4503. (54) Yu, H. G.; Yu, J. G.; Cheng, B.; Lin, J. Synthesis, Characterization and Photocatalytic Activity of Mesoporous Titania Nanorod/Titanate Nanotube Composites. J. Hazard. Mater. 2007, 147, 581-587. (55) Jiang, Z. Y.; Liu, Y. Y.; Jing, T.; Huang, B. B.; Wang, Z. Y.; Zhang, X. Y.; Qin, X. Y.; Dai, Y. Enhancing Visible Light Photocatalytic Activity of TiO2 Using a Colorless Molecule (2-methoxyethanol) due to Hydrogen Bond Effect. Appl. Catal., B 2017, 200, 230-236. (56) Shen, H. Z.; Ie, I. R.; Yuan, C. S.; Hung, C. H. The Enhancement of Photo-Oxidation Efficiency of Elemental Mercury by Immobilized WO3/TiO2 at High Temperatures. Appl. Catal., B 2016, 195, 90-103.

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Figure 1. TEM images of (a) the TiO2 nanoparticles, (b) the Au nanoparticles, (c) the Au-TiO2 nanocomposites and the volume ratio of gold nanoparticles and titanium isopropoxide was 1:4 (The white arrows refer to Au nanoparticles) and (d) the selected area electron diffraction (SAED) pattern of Au-TiO2 nanocomposites.

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Figure 2. XRD patterns. The black line marked the TiO2 powder and the red line marked the Au-TiO2 powder.

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Figure 3. (a-d) SEM images and the corresponding photographs of membranes, (e-f) Images of water contact angle on the membranes surface, and the angle in the figure is the average. (a, e) original PVDF, (b, f) pDA/PVDF, (c, g) TiO2/pDA/PVDF, (d, h) Au-TiO2/pDA/PVDF.

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Figure 4. AFM images of (a) the original PVDF, (b) pDA/PVDF, (c) TiO2/pDA/PVDF and (d) Au-TiO2/pDA/PVDF membranes.

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Figure 5. The FT-IR spectra of the original PVDF, pDA/PVDF, TiO2/pDA/PVDF and Au-TiO2/pDA/PVDF membranes.

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Figure 6. XRD patterns of the original PVDF, pDA/PVDF, TiO2/pDA/PVDF and Au-TiO2/pDA/PVDF membranes. The red box marks the appearance of the new peak.

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Figure 7. The UV-vis absorption/diffuse spectra of pure pDA powder, TiO2 powder, Au-TiO2 powder, original PVDF membranes, pDA/PVDF membranes, TiO2/pDA/PVDF membranes, Au-TiO2/pDA/PVDF membranes and Au-TiO2/PVDF membranes.

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Figure 8. X-ray photoelectron spectroscopy (XPS) wide scans (a) of the original PVDF, pDA/PVDF, TiO2/pDA/PVDF and Au-TiO2/pDA/PVDF membranes; the narrow scans for N 1s (b), Ti 2p3 (c) and Au 4f (d) peaks.

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Figure 9. Photodegration of tetracycline under visible light irradiation (a) and the corresponding kinetic linear simulation curves (b) for the TiO2/pDA/PVDF membranes, the Au-TiO2 powders and the Au-TiO2/pDA/PVDF membranes.

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Figure 10. ESR spectra of the Au-TiO2/pDA/PVDF membranes. (a) DMPO-•O2radical species was detected in methanol, (b) DMPO-•OH was detected in deionized water.

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Figure 11. Photocatalytic Mechanism of degradation of tetracycline for the Au-TiO2/pDA/PVDF membranes.

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Graphical Abstract

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Bioinspired Synthesis of Photocatalytic Nanocomposite Membranes

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